The use of positive displacement hydraulic pumps and motors is well known in the art. When utilizing such a pump, fluctuations within the flow of fluid are created by the reciprocating action of the piston exposed to the suction and the discharge of the pump. These fluctuations of flow are referred to as ripple, thereby producing uneven flow plenums of the fluid.
The primary problem associated with the pressure ripples in such a hydraulic system is noise. The intake and delivery of fluid by positive displacement pumps is unsteady and exhibits fluctuations dependent upon the mechanism of fluid transfer and the number of pumping elements. These periodic flow fluctuations produce pressure fluctuations in the fluid, termed fluid-borne noise, that are transmitted via the fluid throughout the hydraulic system causing structural vibration and air-borne noise. Besides making a significant contribution to the system air-borne noise, high levels of fluid-borne noise can also cause a significant reduction in the life of individual hydraulic components. In fact, it has been predicted that pump life might be reduced by a factor up to three due to the additional effects of fluid-borne noise.
Attempts to reduce the contribution made by the pump to overall noise levels have shown that whereas a high degree of structural isolation may be obtained by flexible mounting of the pump, the isolation of pump pressure ripple is more difficult to achieve.
Previous attempts to reduce the pressure ripple have fallen short. One such attempt utilizes a single cam and piston set that is in fluid communication with both the outlet port and the inlet port of the pump. The drive shaft of the pump rotates the cam thereby driving the pistons to create a fluctuation in the fluid that ideally reduces or offsets the pressure fluctuations created by the pump. This has been found to be restrictive in that the inlet and outlet of the pump have different pressure signatures and would therefore ideally require different ripple compensation profiles. With legislation, in the form of the Health and Safety at Work Act of 1974, Congress has set maximum levels of noise to which an operator in an industrial environment may be subjected. This leads to a serious need for proper and economical noise reduction techniques in an industrial environment. Additionally, military objectives can lead to the need for significant noise reduction within engine and hydraulic systems, since noise is an ideal manner in which an adversary can track and monitor military vehicles such as submarines.
Accordingly, a method and apparatus is needed that overcomes the shortcomings of the prior art by providing a consistent and optimum reduction or cancellation of pressure ripples within the flow of fluid through positive displacement pumps and motors.